The concentration of a particular component in a blood sample, such as glucose, is measured by e.g. an electrode method. In the electrode method, an electrode is brought into contact with a blood sample, and information correlating with the glucose concentration in the blood sample is obtained as an output from the electrode. Based on the output data, the glucose concentration is calculated. Generally, electrode methods are categorized into an equilibrium point method (end point assay) and a differential method (rate assay). In the equilibrium point method, the glucose concentration is computed based on the constant equilibrium value to which the output from the electrode asymptotically approaches. In the differential method, the glucose concentration is computed based on the extreme obtained from the n-th derivative (n is a positive integer) of the output.
It is known that, when whole blood containing blood cells is used as a blood sample in a typical electrode method, the measurement result tends to be lower than the actual value due to the influence of the blood cells. Therefore, to measure the glucose concentration in blood, blood plasma (or blood serum) obtained by removing blood cells from whole blood by centrifugal separation is used as a blood sample. This method, however, is disadvantageous in that the centrifugal separation is necessary for preparing a blood sample, which makes the operation complicated, and that the time taken for the measurement operation, including the preparation of a blood sample, is long. To solve these problems, a method has been proposed in which calculations by the equilibrium point method are correlated with the calculations by the differential method (See Patent Documents 1-3 below).
The method disclosed in Patent Document 1 is conceived based on the fact that, when diluted whole blood is used as a blood sample, the apparent dilution ratio differs from the actual dilution ratio due to the existence of blood cells. This method is based on the assumption that, when diluted whole blood is used as a blood sample, the amount of glucose diffused from within blood cells to the outside before the output corresponding to the maximum is obtained is negligibly small. In the methods disclosed in Patent Documents 2 and 3, the total amount of glucose diffused to the outside of blood cells before the output corresponding to the maximum is obtained is taken into consideration in computing the glucose concentration by the differential method. The total amount of glucose diffused to the outside of the blood cells is related to the hematocrit (proportion of blood cells).
The above-described methods are conceived by attributing the inaccuracy of measurements to the hematocrit and trying to establish a computation technique which is most unlikely to be influenced by the hematocrit. As a result, in the conventional methods, when the glucose concentration in blood is relatively low (e.g. 400 mg/dL or lower), the correlativity with the glucose concentration measured using blood plasma (or blood serum) is improved. However, when the glucose concentration in blood is relatively high (e.g. 500 mg/dL or higher), the correlativity with the glucose concentration measured using blood plasma (or blood serum) is low, and the measurements are still tend to be lower. Therefore, in the above-described method, there is room for improvement with respect to the measurement accuracy in a high concentration range and hence with respect to the measurement range.
Patent Document 1: JP-B-H07-37991
Patent Document 2: JP-A-H09-33533
Patent Document 3: JP-A-H09-318634